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THE ART AND SCIENCE OF IDENTIFYING FLOOD ZONES
Are you planning to develop oceanfront property? Are you responsible for coastal floodplain management? If so, whether you are a coastal developer, geologist, building inspector, or conservation commissioner, you will want to familiarize yourself with how to accurately define the extent of flood zones. In some cases, it is not as simple as reading the latest Flood Insurance Rate Map (FIRM) from the Federal Emergency Management Agency (FEMA). Although this article targets people with experience in these issues, coastal property-owners or potential buyers should be aware that flood zone maps do not always tell the whole story.
FLOOD ZONES DEFINED
USING FIRMS TO IDENTIFY FLOOD ZONES
The base flood elevation (BFE) for a flood zone is the level that flood waters are calculated to reach during a 100-year event. This value is usually printed on a FIRM for each coastal flood zone. FIRM elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD), except for Nantucket, which is referenced to the Half Tide Level Datum of 1934. If your project plan does not use NGVD, convert to a consistent datum. Please note that for V-zones, the BFE is not necessarily a ground elevation. As seen in Figure a, although the BFE for the seaward portion of this V-zone is 16 feet, the velocity conditions do not reach as far landward as the 16-foot ground elevation. Instead, as waves break and wave heights diminish in the land-ward direction, the BFE is reduced.
In general, to delineate specific A-zones, identify the BFE on the FIRM and locate it on the corresponding topographic contour on the project plan (remembering to convert datums if necessary). To delineate V-zones, scale the zone boundary from a known, fixed point, such as a benchmark or road intersection (if scaling from a road on the FIRM, use the center of the road since the lines do not accurately represent road edges), or, at the same scale, overlay the FIRM onto the project plans and trace the zone boundary. When scaling, do not use a shoreline location as a reference point since its position may change over time.
Due to limitations associated with the scale of flood zone mapping, actual V-zone conditions may not match the FIRM boundary for a specific site. Therefore, site topography, land-form type, and nearby transect data from the local FEMA Flood Insurance Study may need to be considered in delineating V-zones for planning purposes. When a detailed study of site conditions is necessary, those without extensive background in flood zone mapping are advised to consult with a professional.
COASTAL SAND DUNESA SPECIAL CIRCUMSTANCE
These updated NFIP regulations define a V-zone or coastal high hazard area as an "area of special flood hazard extending from offshore to the inland limit of a primary frontal dune along an open coast and any other area subject to high velocity wave action from storms or seismic sources." Due to the dynamic nature and erosion potential of primary frontal dunes, such as changes in shoreline configuration from the impact of consecutive storms, "the final rule definition of coastal high hazard area includes all primary frontal dunes. Therefore, the boundary line of the V-zone, at a minimum becomes the landward toe' of the dune." (Federal Register; May 6, 1988)
FEMA provides further clarification of this "540-rule" in its 1995 Guidelines and Specifications for Wave Elevation Determination and V Zone Mapping. Although a dune with a cross-sectional area greater than 540 square feet may provide some level of protection during the 100-year event, by definition the V-zone still includes the entire primary frontal dune. Further analysis is necessary to establish the landward edge of the V-zone if the dune will be overtopped and/or if the cross-sectional area of the primary dune is less than 540 square feet. When the cross-sectional area of the primary dune is less than 540 square feet, the additional analysis involves assuming the dune is removed by erosion during the storm event, and assessing the results of wave runup models to determine how far inland velocity conditions would extend. The FEMA Guidelines provide methodology for evaluating site-specific conditions and delineating V-zones in sand dunes.
Why 540 square feet? FEMA's assessment of dune erosion is based on a statistical analysis of erosion data from actual storm events of different severities. FEMA graphed the cross-sectional area that was eroded from the primary dunes during various coastal storms, then determined the median value of erosion for the data set. For 100-year storms, the median area of erosion was 540 square feet. Using the median means that in 50 percent of the cases, less than 540 square feet is eroded, while more than 540 square feet is eroded in the other 50 percent of the cases. In addition, this analysis does not account for cumulative impacts from multiple storms and long-term erosion. The recent revision of FEMA's Coastal Construction Manual (CCM) addresses these limitations by recommending that a more conservative primary frontal dune measure of 1,100 square feet be used for planning purposes.
In addition to updating the methodology used to identify coastal high hazard areas, FEMA has also provided additional guidance for development in A-zones that may experience coastal storm flooding. A-zones in coastal settings are identified as "Coastal A-zones" in the revised CCM. Although Coastal A-zones experience forces less severe than V-zones, they may still be subject to velocity and wave-related storm conditions capable of causing damage. Therefore, FEMA encourages the application of certain V-zone regulatory requirements within these areas, such as the elevation of new or substantially improved dwellings on open pile foundations.
NEED MORE HELP?
In addition, FEMA has a number of helpful materials available online at www.fema.gov, including: